Electrodeposits of gold are commonly provided on electronic devices to afford excellent wear and corrosion protection, outstanding electrical properties, and other advantageous characteristics. There has developed, however, a trend toward the substitution of palladium and/or palladium/nickel alloys for the gold plate used in the past, and such palladium deposits may most beneficially carry a gold flash overlayer to enhance the wear and corrosion properties. Due to the cost of the precious metals employed, it has become extremely important that means be provided for stripping them from the substrate completely and with minimum contamination, both to remove imperfectly formed deposits and also to permit recovery of the metal values from discarded or worn-out parts.
The prior art discloses means for effecting the removal of gold and/or palladium from substrates. For example, in U.S. Pat. No. 2,185,858 Mason teaches an electrolytic process for dissolving and precipitating gold, which is said to also be applicable for the recovery of palladium. In U.S. Pat. No. 3,819,494, Fountain teaches a method for stripping gold alloy brazing compositions which may contain palladium; the deposit is first subjected to treatment with a composition containing an alkali cyanide and a nitro-substituted aromatic compound, followed by treatment with a nitric acid solution, optionally containing hydrochloric acid. A highly effective formulation for stripping gold and silver is disclosed in Solidum U.S. Pat. No. 3,935,005; the baths are, however, quite ineffective for palladium.
Thus, despite the foregoing prior art disclosures, the need remains for a composition which is capable of simultaneously stripping deposits comprised of palladium and gold in a single step, such as for the removal of gold flash-coated palladium layers from electronic components, and similar parts. It is of course important that any such stripper be capable of operation under practical conditions and at high rates of speed, that it not subject the typical substrate metals to substantial attack, that the make-up composition exhibit a relatively long shelf-life, and that the bath have a capacity for the dissolved metal which is sufficient to avoid the need for frequent replenishment and replacement. Furthermore, it is important that any such formulation be relatively inexpensive, and convenient to package and handle.
Accordingly, it is a fundamental object of the present invention to provide a novel composition which is effective to chemically strip palladium and palladium/nickel alloy deposits from substrates at high rates, without need for electrical energy, and under desirable and practical operating conditions.
An equally important object of the invention is to provide such a formulation which is effective to chemically strip gold simulataneously with such palladium deposits.
Additional objects of the invention are to provide such a novel and relatively economical composition which does not subject the plated substrate metal to undue attack, which has a good capacity for the dissolved metals, which can readily and effectively be rejuvenated to extend its useful life, which can be made up with minimum risk to the operator, and which is conveniently packaged and exhibits a relatively long shelf-life
Further objects of the invention are to provide novel solutions comprised of such formulations, and to provide novel methods by which the solutions are employed in stripping operations, and particularly to strip deposits comprised of palladium and gold, in a single step.